1. Field of the Invention
The present invention relates to a receiving apparatus and receiving method, and more particularly, to a receiving apparatus and receiving method for receiving and demodulating signals transmitted from a base station by spread spectrum communication.
2. Description of the Related Art
In mobile communication, code division multiple access (CDMA) communication is used widely. In code division multiple access (CDMA) communication, information sent via a plurality of channels or information sent from users is multiplexed with a spreading code and is transmitted via a radio line.
A conventional receiving apparatus 2, as shown in FIG. 8, includes an antenna 21, a Radio Frequency (RF) unit 22, an analog-to-digital converter (A/D) 23, a cell search unit 24, a path search unit 25, and a RAKE receiving unit 26. The conventional receiving apparatus 2 operates as follows.
A plurality of signals caught with the antenna 21 is frequency-converted by the RF unit 22, and converted into digital signals by the A/D 23. The digital signals are supplied to each of the cell search unit 24, the path search unit 25, and the RAKE receiving unit 26.
The cell search unit 24 detects a neighboring cell and measures its received electric power, and detects a scrambling code and frame timing of a base station (not shown).
The path search unit 25 receives the digital signals output from the A/D 23, searches them for multi-path, and outputs each of path timings.
The RAKE receiving unit 26 refers to the path timings (information) output from the path search unit 25, performs a despreading process on each path of the digital signals which are output from the A/D 23, demodulates into the original data from symbol data obtained by despreading, and combines and outputs demodulated results.
Namely, this conventional receiving apparatus 2 has selected a high level path detected in the range of the search detection window assumed beforehand as an effective path at just time of detection of a neighboring cell (base station candidate). However, for example, in a surrounding environment which has only a direct wave, delay profile will be created vainly and power consumption will be wasted. Moreover, when the search detection window was narrowed for reduction of power consumption, degradation of receiving quality was caused without a required path being combined, depending on the surrounding environment.
As an example which solves such a problem, there is a radio receiving apparatus capable of performing a highly precise path search (see, JP 2003-110459). This apparatus finely searches only in the neighborhood of a path candidate about a PRACH message portion by using a phase of path candidate searched when a preamble portion of Physical Random Access Channel (PRACH) was received.
Moreover, there is a mobile communication terminal capable of performing a path search exactly about a range over which effective paths are distributed (see, JP 2002-141833). This terminal searches for a center of gravity of a distribution of a plurality of paths suitable for reception, and performs path search in the range based on the center of gravity.
Furthermore, there is a radio receiving apparatus capable of changing path search window width based on an amount of delay (see, JP 2002-111548). This apparatus judges an amount of delay of a path with amount of maximum delay from a path timing which has a peak level, and presupposes a window width of a path search based on the amount of delay.
What is more, there is a receiver of the spread spectrum communication which sets up two path search windows (see, JP 2001-223613). This apparatus includes a tracking window which performs a path search of near the center of search window within the limits, and a search window which performs a path search by later domain time sharing.
However, in the above-mentioned conventional receiving apparatus, since cell search processing and path search processing operate independently, there is a following problem. That is, it is necessary to perform reception processing after newly performed path search processing, when communication were started between a new base station.